1. Field of the Invention
The present invention relates to the prevention and/or inhibition of reverse engineering of digital integrated circuits, and more particularly to covertly turning a transistor permanently ON via a double-polysilicon layer CMOS process with buried interconnects and to an integrated circuit structure comprising the same.
2. Description of the Related Art
The design, development and manufacturing efforts pertaining to semiconductor integrated circuits involve the understanding of complex structures, processes and manufacturing techniques involving smaller and smaller electronic circuitry. Efforts to be able to achieve such understanding and establish successful design, development and production manufacturing of such integrated circuits involve many man-hours of highly skilled professionals and considerable expense.
On the other hand, to avoid costly man-hours and other significant expenses some developers resort to reverse engineering practices wherein existing devices are taken apart, probed and otherwise examined to determine the physical structures of the resultant integrated circuit under review for subsequent copying. This reverse engineering, which typically relies primarily on obtaining planar optical image of the circuit, in essence attempts to by-pass typical product development efforts and expenses by studying and copying a competitive product.
Various approaches have been developed in an attempt to thwart such reverse engineering efforts, particularly in the field of semiconductor integrated circuits.
For example, U.S. Pat. No. 5,866,933 in the name of the same inventors of the present application teaches how transistors in a CMOS circuit can be connected by hidden lines between the transistors, via modifying the P+ and N+ source/drain masks. These implanted interconnections are further used to make a 3-input AND and OR circuit look substantially the same.
Moreover, U.S. Pat. No. 5,783,846 in the name of the same inventors of the present application teaches a further modification in the source/drain implant masks so that the implanted connecting lines between transistors have a gap inserted, the length of which is approximately the length of the feature size of the CMOS technology being used. If the gap is xe2x80x9cfilledxe2x80x9d with one kind of implant (depending on the implanted connecting line being P or N) the line conducts. But, if the gap is filled with the other kind of implant the line does not conduct. These gaps are called xe2x80x9cchannel blocksxe2x80x9d. Thereby the reverse engineer must determine connectivity on the basis of resolving the n or p implant at the minimum feature size of the channel block. Moreover, transistor sizes and metal connection routings are modified, to eliminate keys by which the reverse engineer can find inputs, outputs, gate lines etc. as keys to the circuit functionality.
However, integrated circuits protected with the art taught and referenced above look different from standard integrated circuits produced with the same CMOS process. This gives the reverse engineer an indication that something in the circuit at hand is different.
Designing a circuit to appear as one function, but operate as another, is a good way to protect a circuit against reverse process engineering. The present invention applies to the following genre of CMOS fabrication processes. The process according to the present invention employs at least two polysilicon layers. Such is the case in virtually all CMOS processes that manufacture DRAMs and EEPROMs. Moreover, the process according to the present invention makes use of a buried contact process.
The process according to the present invention provides selected transistors within the integrated circuit that are permanently ON despite the fact that they will look to the reverse engineer as normally functioning transistors.
In this way, the present invention overcomes the problems of the prior art by making use of a buried contact technique to make permanently turned-ON transistors in a double polysilicon CMOS process. In particular, as further disclosed in the present invention, both polysilicon layers can be deposited before source and drain implants.
A buried contact is a known structure in integrated circuit fabrication technology, firstly developed in the late 70""s with nMOS circuits. A buried contact provides a direct connection between polysilicon, normally the gate of a MOS transistor, and the source/drain region of a MOS transistor. To create buried contacts, contact openings are masked and etched after gate oxide growth and before deposition and patterning of polysilicon. In this way the polysilicon over the source and/or drain regions can make direct contact to the source and/or drain regions by means of the buried contact openings. See for example S. M. Sze, VLSI Technology, McGraw-Hill, pages 461-478.
The present invention makes use of buried contacts to create permanently ON transistors that look identical to normal transistors. As a result, circuits can be designed to appear as performing one function but in fact perform another. Neither the ON transistors nor the other transistors require that polysilicon be deposited after source and drain implantation, as required by the prior art.
This also means that the present invention, which can be used with the art referenced above, or by itself, does not change the overall appearance of the integrated circuit. That is because the turning of the transistor permanently ON is accomplished in a covert manner.
According to a first aspect of the present invention, a permanently-ON MOS transistor is provided, comprising: a silicon source region of a first conductivity type; a silicon drain region of the first conductivity type; a silicon well region of a second conductivity type, in which said source region and drain region are buried; a silicon contact region of the first conductivity type, buried in the well region, said contact region contacting said source region and said drain region; a first gate insulating layer selectively placed over the silicon source region and the silicon drain region; a second gate insulating layer selectively placed over the first gate insulating layer and over the silicon contact region; and a polysilicon gate region placed over the second gate insulating layer.
According to a second aspect of the present invention, an integrated circuit structure for MOS-type devices is provided, comprising at least one permanently-ON MOS transistor, said transistor including: a first silicon source region of a first conductivity type; a first silicon drain region of the first conductivity type; a first silicon well region of a second conductivity type, in which the first silicon source region and the first silicon drain region are buried; a silicon contact region of the first conductivity type, buried in the well region, the contact region contacting the silicon source region and the silicon drain region; a first gate insulating layer selectively placed over the silicon source region and the silicon drain region; a second gate insulating layer selectively placed over the first gate insulating layer and over the silicon contact region; and a first polysilicon gate region placed over the second gate insulating layer.
According to a third aspect of the present invention, a process is provided for forming a permanently-ON MOS transistor comprising the steps of: providing a silicon well region of a first conductivity type; depositing a first insulating layer over the silicon well region of the first conductivity type; removing a portion of said deposited first insulating layer; forming a buried silicon region of a second conductivity type within the silicon well region of the first conductivity type, under the removed portion of said deposited first insulating layer; depositing a second insulating layer over the first insulating layer and over the buried silicon region of the second conductivity type; forming a polysilicon gate region over the second insulating layer; and forming a source region of the second conductivity type and a drain region of the second conductivity type within the buried silicon well region, said source region and drain region contacting said buried silicon region of the second conductivity type.